This paper is a contribution to the characterization of permeability to gas of unsaturated GCLs, with applications in cover liners of municipal solid waste landfills. Conducting such tests at low gas pressure gradient limits the risk of moisture distribution change in the bentonite, which has been observed under high pressure gradients. However, a consequence of a low pressure gradient is a small gas flow rate that is difficult to measure with good accuracy. Therefore, a solution may be the use of transient state tests, like the falling pressure test adapted to GCLs permeability characterization. This paper shows the influence of the main parameters related to the test and to the material tested (natural or activated calcium bentonite, moisture content, presence or not of small cracks in the bentonite). The results obtained showed good repeatability for gas differential pressures up to 40 kPa for the test conditions employed in this work. The results showed that the gravimetric moisture content of the GCL necessary to attain a certain permeability value depends on the bentonite nature, which was not observed in terms of volumetric moisture content. However, other factors showed to be more important than the nature of bentonite in the GCL permeability to gas: the desiccation due to the gas flow can increase significantly the permeability, which compromise the GCL performance as a gas barrier.